A rotation angle detection device, for example, detects a rotation angle of a rotating shaft of a motor and controls the position of a movable body of a machine tool or the like which is driven by the motor according to a detected value of the rotation angle. In order to control the position of the movable body with high accuracy over a wide range, the rotation angle detection device is desirably capable of detecting the motor rotating shaft as an absolute rotation angle over multiple turns.
As such a rotation angle detection device, Japanese Patent Laid-Open No. 2010-44055 (corresponding to U.S. patent application Ser. No. 12/168,151) discloses an inductive multi-turn encoder. The inductive multi-turn encoder 10 includes geared circular disks 41 to 46 connected in series and in multiple stages and realizes detection of a multi-turn angle by detection of rotation angles of the disks 42, 44, and 46. A range of a multi-turn angle which can be detected by the inductive multi-turn encoder disclosed in the prior art literature is determined by a reduction ratio of the disk 46 in the final stage to an input rotatable shaft. A rotatable shaft 20 as the input rotatable shaft is mechanically connected to the circular disk 4I with a gear reduction ratio of I:4. The subsequent geared circular disks 4I″ and 42, 42″ and 43, 43″ and 44, 44″ and 45, and 45″ and 46 are mechanically connected with respective gear reduction ratios of 1:4. By making such 1:4 gear reductions in six stages, the multi-turn encoder 10 can obtain a multi-turn detection range up to 4096 revolutions.